Wastewater treated by wastewater treatment systems contain soluble, partially soluble and insoluble in and contaminates. materials may be decomposable, partially decomposable or not decomposable. Decomposable and partially decomposable materials are referred to as biodegradable, that is, the material may be biologically broken down or stabilized by bacterial action. Wastewater treatment systems are designed to provide controlled decomposition of wastes to reduce pollution, health hazards and offensive odors.
Decomposable material is stabilized in wastewater treatment systems by bacteria, protozoa, and other microorganisms. Bacterial consumption of material, creating energy and reproducing bacterial cells, is the foundation of activated sludge wastewater treatment.
Conventional wastewater treatment systems may include pretreatment, primary treatment, secondary treatment, and advanced treatment.
Pretreatment includes screening, comminuting (mechanical cleaning of screens by shredding solids to a size which can pass through screen openings), degritting, and grease and scum removal.
Primary treatment includes removal of suspended solids from wastewater by clarification and skimming, typically involving a tank or channel, reducing flow velocity, settling of heavier solids and skimming of relatively light solids. Primary treatment may include anaerobic digestion processes, aerobic digestion processes, or a combination thereof. Primary treatment systems typically include sludge collection mechanisms, sludge suction devices, grit removal devices, and sludge dewatering devices to reduce the volume of sludge to be disposed.
Secondary treatment systems are typically aerobic systems including an aeration phase and a clarification phase. Secondary treatment systems may follow primary treatment, or in some applications, be utilized in lieu of primary treatment systems. Secondary treatment systems typically include an aeration basin, an air distribution system, a clarifier, sludge collection mechanisms, and sludge removing devices.
Advanced treatment includes further removal of suspended and dissolved organic solids by means including filtration, removal of pathogens and chloroforms by oxidation, chlorination or heating, precipitation of minerals, adsorption or other methods.
In the activated sludge process of primary or secondary treatment, microorganisms are contained in an activated sludge and mixed with incoming wastewater; the wastewater providing food for the microorganisms. Such mixing is accomplished in an aeration tank or channel. In the aerobic activated sludge process, oxygen is intimately mixed with the activated sludge and the wastewater, the microorganisms converting suspended organic solids into energy, carbon dioxide, water, and additional microorganism cells. The aerobic activated sludge process therefore includes (i) mixing of wastewater, activated sludge, and oxygen in an aeration tank, (ii) conversion of suspended organic solids, (iii) settling of activated sludge in the clarifier, (iv) returning the activated sludge to the aeration tank for further treatment (v) removing purified liquor from the clarifier, and (vi) removal and disposal of the final, inert sludge.
Current technology used in the design of activated sludge wastewater treatment plants provides a high amount of process control and consistent treatment of wastewater for municipal and industrial applications. The conventional design of such plants, however, requires the use of a large number of mechanical subsystems including pumps, blowers, gears, chains, and associated mechanical elements. The large quantity of mechanical parts makes such conventional systems expensive to construct and maintain as well as difficult to operate.
The disadvantages of conventional designs using a large number of mechanical subsystems are particularly difficult for smaller communities which cannot afford the capital outlay for an activated sludge treatment plant and would have difficulty in reliably maintaining the mechanical performance and/or recruiting qualified maintenance personnel, resulting in inefficiently operated plants producing poor quality effluent.
Baldespino U.S. Pat. No. 3,220,706 discloses a sewage treatment system comprising in combination an aeration unit including a generally circular tank, a tangentially disposed adjustably-sized sewage inlet located along the wall of the tank, an overflow discharge line, a circulating pump, vertically adjustable air inlet means requiring blowers or compressors in the middle of said tank spaced from the bottom of said tank, blowers and a liquid spray ring around the upper interior portion of the tank having downwardly-directed orifices. Sewage introduced into the tank through tangential pump discharge inlets is caused to rotate. Compressed air from the air inlet means induces movement in the sewage vertically at right angles to the first rotation thereby resulting in rolling, swirling, action of the sewage with intimate exposure of the sewage with the oxygen in the air. The aeration tank includes an inclined tank bottom, the tank bottom inclined upwardly and outwardly from the center with a sump at the lower level. Through tangential flow of the sewage introduced, and vertical flow induced by the aeration ring, the Baldespino disclosure seeks to obtain rolling turbulent flow within the aeration unit. Grit, such as sand, free from solid constituents, is accumulated at the center of the tank (Column 6, lines 7-16, Column 3, lines 1-3). The Baldespino disclosure includes a sump, an aeration tank, a settling tank, a sludge digestion tank, and various valves, blowers, and pumps connecting the various tanks.
Hell, et al. U.S. Pat. No. 4,629,565 discloses a method for purifying wastewater in a settling plant wherein the final settling tank is arranged inside a scavenger basin. The aeration of waste is separated from the aeration of the return sludge, the waste and sludge flows being mixed before entering the scavenging basin.
Mandt U.S. Pat. No. 4,596,658 describes a system for removing clarified wastewater in a sequential batch-type plant including a subsurface decanter manifold extending horizontally along the discharge wall of the batch plant tank, the decanter manifold including angled, parallel and overlapping members limiting flow into the decanter manifold through a horizontally extended orifice defined by said overlapping members.
Schneider et al. U.S. Pat. No. 4,452,700 describes a process for the performance and control of chemical or biochemical process cycles in two or more reaction basins, the basins arranged concentrically to one another, the basins having connection between one another through which fluid passes. The contents of the basins are caused to assume a horizontal rotational flow, such rotational flow induced by propellers. Connections between the basins may be in the form of weirs, flaps, deflectors, wall openings, or other passage openings arranged in the container wall of the inner basin. The Schneider disclosure includes gassing using an auxiliary aerator in the first stage and in the second stage with recirculation of the fluid between the first stage and the second stage.
Reed U.S. Pat. No. 4,443,338 discloses a method for converting a reactor basin utilizing an activated sludge process from a plug flow or complete mix configuration to an oxidation ditch configuration, the method including forming an endless channel within the basin, placing a barriered circulator/aerator within the endless channel, said circulator/aerator comprising a barrier means, flow passage means, pump means, and aeration means for dispersing an oxygen containing gas into the liquor stream and selectively and independently operating the pump means and aeration means.
As indicated by the foregoing references and the references cited therein, the current technology includes a wide array of wastewater treatment subsystems and combinations thereof. As the references indicate, wastewater treatment subsystems typically include numerous mechanical, electrical, and hydraulic components.
It is an object of the present invention to provide a wastewater treatment system wherein efficiency of mechanical systems and energy is accomplished by improved utilization of hydraulic characteristics and physical properties of particulate motion and settling, the process relying on a single pumping unit to induce all liquid and oxygen flow through the aeration and clarification processes thereby minimizing the reliance on mechanical components and subsystems.
It is a further object of the present invention to provide a wastewater treatment system having minimal control variables wherein process control requires minimal operator input.
It is a further object of the present invention to provide an efficient wastewater treatment system wherein activated sludge age may be varied to optimize metabolic process independent of plant throughput.
It is a further object of the present invention to provide a wastewater treatment system which minimizes the number of mechanical components in the system.
It is a further object of the present invention to provide an efficient wastewater treatment system wherein the rate of wasting (thickening and aging) may be varied independent of plant throughput.
It is a further object of the present invention to provide a relatively efficient wastewater treatment system.
It is further object of the present invention to provide a wastewater treatment system providing relatively large capacity for wastewater treatment in a relatively small area.
The foregoing and other objects are accomplished by a wastewater treatment system that includes a circular aeration basin and an annularly extending clarifier channel. Circular flow of a mixture of untreated wastewater and return sludge is induced into the aeration basin by tangentially mounted Venturi aerators. The aerators are spaced along the circumference of the basin and fed through a common manifold, which manifold is located within the basin. The hydraulic circulation within the aeration basin establishes a vortex within the basin, the flow being generally whirlpool-like. Such flow provides a controlled, relatively less turbulent area at the central axis of the basin.
The Venturi aerators include nozzles attached to the common manifold and supplied with environmental air. The liquid velocity through the Venturi nozzles creates fine bubble dispersion of air within the mixed liquid, creating efficient oxygen transfer to the mixed liquid and avoiding clogging problems often associated with conventional systems.
The clarifier comprises an annular chamber contiguous with the outside wall of the aeration basin. The clarifier inlet is hydraulically connected to the aeration basin through openings provided in the aeration basin wall; the clarifier inlet includes means to dissipate turbulence.
Surface skimmers are provided in spaced relationship around the clarifier, such skimmers being hydraulically connected to the aeration basin for skimming and returning surface scum to the aeration basin.
A sludge removal manifold extends within the clarifier with radially-spaced, valve-controlled suction arms. The spaced suction arms provide for precise control of sludge blanket depth and sludge age independent of sludge return rate.
Process sludge withdrawn through the suction arms is mixed with raw wastewater and returned to the aeration basin.
Clarified fluid is withdrawn from the annular clarifier channel through a trough and orifices provided at the end of the clarifier channel.
The circular flow within the aeration basin and the relatively less-turbulent flow in the central area of the basin allows process sludge to accumulate in said central area. Such sludge may be maintained for relatively long periods of time to allow reduction of remaining volatiles. A drain provided centrally in the aeration basin allows for draw-off of such digested sludge utilizing the hydraulic head differential in the tank, draw-off being accomplished by valve control piping connected to the drain.